Martensitic stainless steel seamless tube for oil country tubular goods and manufacturing method thereof

ABSTRACT

A martensitic stainless steel seamless tube for oil country tubular goods includes a yield strength of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/073918, with an international filing date of Dec. 24, 2008 (WO 2010/026672 A1, published Mar. 11, 2010), which is based on Japanese Patent Application No. 2008-226578, filed Sep. 4, 2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a martensitic stainless steel seamless tube for oil country tubular goods, more particularly, to a seamless steel tube for OCTG which possesses both of high strength of 95 ksi (655 MPa) in terms of yield strength (YS) and excellent low-temperature toughness, and a manufacturing method thereof.

BACKGROUND

Recently, in view of sharp rise of oil price and exhaustion of petroleum expected in near future, the development of oil wells which have not been considered as targets of development because of extreme depths thereof, the development of oil wells or gas fields in a severe corrosion environment which contains carbon dioxide gas, chlorine ions or the like, the development of oil wells in a severe drilling environment such as a cold district or a seabed and the like have been promoted. A steel tube for an oil well used in such a severe environment is required to be made of a material which possesses high strength, excellent corrosion resistance and excellent toughness.

Conventionally, in oil wells and gas fields which contain carbon dioxide CO₂, chlorine ions Cl⁻ or the like, a 13%-Cr martensitic stainless steel tube has been popularly used as an oil country tubular good used in digging.

For example, JP-A-2002-363708 proposes martensitic stainless steel which contains 0.01 to 0.1% of C, 9 to 15% of Cr and 0.1% or less of Ni. Although the stainless steel exhibits relatively high C content so that the stainless steel possesses high strength, the stainless steel possesses high toughness. Accordingly, the stainless steel is preferably used for manufacturing oil country tubular goods. According to a technique disclosed in JP-A-2002-363708, it is estimated that by reducing a quantity of carbide present in a prior austenite grain boundary to 0.5 volume % or less, by setting a maximum minor axis of carbide to 10 to 200 nm and by setting a ratio between average Cr concentration and average Fe concentration in carbide to 0.4 or less, the precipitation of M₂₃C₆-type carbide is suppressed and the precipitation of M₃C-type carbide is accelerated, thus largely improving toughness. To adjust the structure and the composition of such carbide within desired ranges, according to the technique disclosed in JP-A-2002-363708, the stainless steel is subject to tempering at a temperature of 450° C. or below in such a manner that the stainless steel is subject to air cooling (standing to cool) after hot working, is held in air cooling (standing to cool) after solution treatment, or is subject to air cooling (standing to cool) after solution treatment.

However, when the stainless steel is subject to low-temperature tempering at 450° C. or below using the technique disclosed in JP-A-2002-363708, straightening which follows the tempering treatment is performed at a low temperature. Hence, processing strain is induced in the stainless steel in straightening, thus giving rise to a drawback that irregularities in steel tube properties, particularly irregularities in yield strength YS are increased.

It could therefore be helpful to provide a seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and excellent low-temperature toughness, and a method of manufacturing the seamless steel; tube in a stable manner. “Excellent low-temperature toughness” implies a case where a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below.

SUMMARY

We thus provide:

-   -   (1) A martensitic stainless steel seamless tube for oil country         tubular goods possessing both of high strength of yield strength         of 95 ksi or more and excellent low-temperature toughness in         which a fracture transition temperature vTrs in a Charpy impact         test is −40° C. or below, the seamless tube having the         composition which contains, by mass %, 0.020% or less C, 10 to         14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe         and unavoidable impurities as a balance, and having the         structure where a precipitated Nb quantity is 0.020% or more in         terms of Nb.     -   (2) A martensitic stainless steel seamless tube for oil country         tubular goods possessing both of high strength of yield strength         of 95 ksi or more and excellent low-temperature toughness in         which a fracture transition temperature vTrs in a Charpy impact         test is −40° C. or below, the seamless tube having the         composition which contains, by mass %, 0.020% or less C, 1.0% or         less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S,         0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb,         0.05% or less N, and Fe and unavoidable impurities as a balance,         and having the structure where a precipitated Nb quantity is         0.020% or more in terms of Nb.     -   (3) In the martensitic stainless steel seamless tube for oil         country tubular goods having the constitution (1) or (2), the         seamless steel tube has the composition which contains the Nb         such that a following formula (I) is satisfied in terms of C, Al         and N:

C−31/4Nb+7/6N−9/4Al≦−0.30  (1).

-   -    (C, Nb, N, Al: contents of respective elements (mass %)). In         the stainless steel having the constitution (1), it is treated         that Al content is 0 mass %.     -   (4) In the martensitic stainless steel seamless tube for oil         country tubular goods having any one of the constitutions (1) to         (3), the composition of the seamless tube contains, by mass %,         in addition to the above-mentioned composition, one or two kinds         of elements selected from 2.0% or less Cu, and 2.0% or less Mo.     -   (5) In the martensitic stainless steel seamless tube for oil         country tubular goods having any one of the constitutions (1) to         (4), the composition of the seamless tube contains, by mass %,         in addition to the above-mentioned composition, one kind, two or         more kinds of elements selected from 0.20% or less V, 0.10% or         less Ti and 0.005% or less B.     -   (6) A manufacturing method of a martensitic stainless steel         seamless tube for oil country tubular goods possessing both of         high strength and excellent low-temperature toughness, the         method comprising: quenching in which a seamless tube having the         composition which contains, by mass %, 0.020% or less C, 10 to         14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe         and unavoidable impurities as a balance is heated at a quenching         temperature of A_(c3) transformation temperature or above and,         thereafter, the seamless tube is cooled to a temperature range         of 100° C. or less from the quenching temperature at a cooling         rate of air cooling or more; and tempering which follows the         quenching and in which the seamless tube is heated at a         tempering temperature of 550° C. or more and A_(c1)         transformation temperature or below and is cooled.     -   (7) A manufacturing method of a martensitic stainless steel         seamless tube for oil country tubular goods possessing both of         high strength and excellent low-temperature toughness, the         method comprising: quenching in which a seamless tube having the         composition which contains, by mass %, 0.020% or less C, 1.0% or         less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S,         0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb,         0.05% or less N, and Fe and unavoidable impurities as a balance         is heated at a quenching temperature of A_(c3) transformation         temperature or above and, thereafter, the seamless tube is         cooled to a temperature range of 100° C. or less from the         quenching temperature at a cooling rate of air cooling or more;         and tempering which follows the quenching in which the seamless         tube is heated at a tempering temperature of 550° C. or more and         A_(c1) transformation temperature or below and is cooled.     -   (8) In the manufacturing method of a martensitic stainless steel         seamless tube for oil country tubular goods having the         constitution (6) or (7), the seamless steel tube has the         composition which contains the Nb such that a following         formula (I) is satisfied in terms of C, Al and N:

C−31/4Nb+7/6N−9/4Al≦−0.30  (1).

-   -    (C, Nb, N, Al: contents of respective elements (mass %)). In         the stainless steel having the constitution (6), it is treated         that Al content is 0 mass %.     -   (9) In the manufacturing method of a martensitic stainless steel         seamless tube for oil country tubular goods having any one of         the constitutions (6) to (8), the composition of the seamless         tube contains, by mass %, in addition to the composition, one or         two kinds of elements selected from 2.0% or less Cu, and 2.0% or         less Mo.     -   (10) In the manufacturing method of a martensitic stainless         steel seamless tube for oil country tubular goods having any one         of the constitutions (6) to (9), the composition of the seamless         tube contains, by mass %, in addition to the composition, one         kind, two or more kinds of elements selected from 0.20% or less         V, 0.10% or less Ti and 0.005% or less B.     -   (11) In the manufacturing method of a martensitic stainless         steel seamless tube for oil country tubular goods having any one         of the constitutions (6) to (10), straightening is performed in         a temperature range of 450° C. or more during cooling after the         tempering.     -   (12) A martensitic stainless steel seamless tube for oil country         tubular goods obtained by the manufacturing method of a         martensitic stainless steel seamless tube for oil country         tubular goods described in (11), wherein an increment ΔYS of         yield strength by the straightening is 15 MPa or less.

Hot straitening of our stainless steel tube can be realized. Accordingly, even when straitening is performed, the increase of yield strength is small so that it is possible to manufacture easily and stably a seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and excellent low-temperature toughness where a fracture transition temperature vTrs is −40° C. or below, thus acquiring remarkable industrial advantageous effects.

DETAILED DESCRIPTION

We studied the influence of content composition and heat treatment conditions exerted on a change of toughness brought about by increasing strength of a 13Cr martensitic stainless steel tube. As a result of the study, we found that it is possible to prevent the deterioration of toughness attributed to grain boundary precipitation of M₂₃C₆ type Cr-based carbide by adopting a content system in which a C content is limited to 0.02% or less, a Cr content is set to a value which falls within approximately 10% to 14% Cr where corrosion resistance is not deteriorated, Ni content is set to a relatively low value of 3% or less, and a relatively large quantity of, that is, 0.03% or more of Nb is contained in the content system. We also found that, due to such content composition, even when tempering is applied to the steel tube at a high temperature of 550° C. or more after quenching, it is possible to manufacture the steel tube which ensures high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and possesses high toughness where vTrs is −40° C. or below. Further, a straitening temperature can be set to a high temperature of 450° C. or more. Hence, the increase of yield strength YS after straitening is decreased to 15 MPa or less.

Reasons for limiting the composition of seamless steel tube for OCTG are explained. Hereinafter, mass % is simply described as % unless otherwise specified.

The seamless steel tube for OCTG is a martensitic stainless steel seamless tube which adopts the composition which contains 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition. The martensitic stainless steel seamless tube may adopt the composition which contains 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition.

C: 0.020% or less

C is an important element relating to strength of martensitic stainless steel. Although it is desirable that the stainless steel contains 0.003% or more of C to ensure desired high strength, when a content of C exceeds 0.020%, toughness and also corrosion resistance are liable to be lowered. Accordingly, the content of C is limited to 0.020% or less. From a view point of stable assuring of strength and toughness, the content of C is preferably limited to a value which falls within a range from 0.003% to 0.015%.

Cr: 10 to 14%

Cr is an element which enhances corrosion resistance due to the formation of a protective film, and is an element which effectively contributes to the enhancement of CO₂ corrosion resistance and CO₂ stress corrosion cracking resistance. When the stainless steel contains 10% or more of Cr, the stainless steel can ensure required corrosion resistance as oil country tubular goods. Hence, 10% is set to a lower limit of Cr content. On the other hand, when the stainless steel contains a large quantity of Cr exceeding 14%, ferrite is easily formed so that the addition of a large quantity of expensive austenite forming element becomes necessary for ensuring stability of a martensite phase or for preventing the lowering of hot workability whereby the content of Cr exceeding 14% is economically disadvantageous. Accordingly, the content of Cr is limited to a value which falls within a range from 10 to 14%. From a view point of ensuring the more stable structure and the more stable hot workability, the content of Cr is preferably limited to a value which falls within a range from 10.5 to 11.5%.

Ni: 3% or less

Ni is an element which has a function of strengthening a protective film, and enhances corrosion resistance such as CO₂ corrosion resistance. Although it is desirable that the stainless steel tube contains 0.1% or more of Ni to acquire such an advantageous effect, when the content of Ni exceeds 3%, this leads to only a sharp rise of a manufacturing cost. Accordingly, the content of Ni is limited to a value which falls within a range not more than 3%. The content of Ni is preferably limited to a value which falls within a range from 1.5 to 2.5%.

N: 0.05% or less

N is an element which remarkably enhances pitting corrosion resistance, and such an advantageous effect becomes outstanding when the content of N becomes 0.003% or more. On the other hand, when the content of N exceeds 0.05%, various nitrides are formed thus lowering toughness. Accordingly, the content of N is limited to 0.05% or less. The content of N is preferably limited to a value which falls within a range from 0.01 to 0.02%.

Nb: 0.03 to 0.2%

Nb is an important element. Nb is an element which forms a carbide, and increases strength of steel through precipitation strengthening by Nb carbide. Further, Nb plays an important role for preventing the grain boundary precipitation of M₂₃C₆ type Cr carbide, thus enhancing toughness. To acquire such an advantageous effect, it is necessary to set the content of Nb to 0.03% or more, and more preferably to a value which exceeds 0.03%. Further, from a view point of acquiring higher strengthening and higher toughness, it is preferable to set the content of Nb to 0.06% or more. On the other hand, when the content of Nb exceeds 0.2%, toughness is lowered. Accordingly, the content of Nb is limited to a value which falls within a range from 0.03 to 0.2%. The content of Nb is preferably limited to a value which falls within a range from 0.03% to 0.15%. The content of Nb is more preferably limited to a value which falls within a range from 0.06 to 0.15%.

Although the above-mentioned components are basic components of the stainless steel, it is preferable to adopt the composition which contains 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, and 0.10% or less Al in addition to these basic components as the basic composition.

Si: 1.0% or less

Si is an element which functions as deoxidizing agent in a usual steel making process. Although it is desirable to set the content of Si to 0.1% or more, when the content of Si exceeds 1.0%, toughness is lowered and cold workability property is also lowered. Accordingly, the content of Si is limited to 1.0% or less. The content of Si is preferably limited to a value which falls within a range from 0.1 to 0.3%.

Mn: 0.1 to 2.0%

Mn is an element which increases strength of the stainless steel. Although it is desirable that the content of Mn is 0.1% or more to allow the stainless steel to ensure strength necessary for a steel tube for oil country tubular goods, when the content of Mn exceeds 2.0%, toughness is adversely influenced by Mn. Accordingly, the content of Mn is limited to a value which falls within a range from 0.1 to 2.0%. The content of Mn is preferably limited to a value which falls within a range from 0.5 to 1.5%.

P: 0.020% or less

P is an element which deteriorates corrosion resistance such as CO₂ corrosion resistance. Hence, it is desirable to reduce the content of P as much as possible. However, the extreme reduction of the content of P pushes up a manufacturing cost. As a range of the content of P which can realize the relatively inexpensive industrial manufacture of the stainless steel, and prevents the deterioration of corrosion resistance such as CO₂ corrosion resistance, the content P is limited to 0.020% or less. The content of P is preferably limited to 0.015% or less.

S: 0.010% or less

S is an element which remarkably deteriorates hot workability in a tube manufacturing step. Although it is desirable to decrease the content of S as much as possible, the tube can be manufactured in a usual step by decreasing the content of 5 to 0.010% or less. Hence, the content of S is limited to 0.010% or less. The content of S is preferably limited to 0.003% or less.

Al: 0.10% or less

Al is an element which possesses a strong deoxidizing action. To acquire such an advantageous effect, it is desirable that the stainless steel contains 0.001% or more of Al. However, when the content of Al exceeds 0.10%, Al adversely influences toughness. Accordingly, the content of Al is limited to 0.10% or less. The content of Al is preferably limited to 0.05% or less.

It is preferable that the content of Nb falls within the above-mentioned content range and, further, a following formula (I) is satisfied in view of the relationship between the content Nb and the contents of C, Al and N:

C−31/4Nb+7/6N−9/4Al≦−0.30  (1).

(C, Nb, N, Al: contents of respective elements (mass %)).

In the formula (I), when an addition content of Al is at a level of unavoidable impurities, the calculation is made assuming the content of Al as 0 mass %.

When the Nb content does not satisfy the above-mentioned formula (I), the stainless steel cannot possess both of desired high strength (yield strength: 95 ksi or more) and high toughness (fracture transition temperature vTrs in a Charpy impact test is −40° C. or below).

In addition to the above-mentioned basic components, the stainless steel may include one or two kinds of components in a group A and a group B described hereinafter:

-   -   Group A: one kind, or two kinds selected from 2.0% or less Cu,         2.0% or less Mo.     -   Group B: one kind or two or more kinds selected from 0.20% or         less V, 0.10% or less Ti, 0.005% or less B.         Group A: one kind or two kinds selected from 2.0% or less Cu,         2.0% or less Mo.

Both Cu and Mo are elements which have a function of enhancing corrosion resistance and the stainless steel may selectively contain these elements when necessary.

Cu is an element which has a function of strengthening a protective film thus enhancing pitting resistance, and it is desirable to set the content of Cu to 0.2% or more to acquire such an advantageous effect. On the other hand, when the content of Cu exceeds 2.0%, Cu or a part of Cu compound precipitates thus lowering toughness. Accordingly, when the stainless steel contains Cu, the content of Cu is preferably limited to 2.0% or less. The content of Cu is more preferably limited to a value which falls within a range from 0.2 to 1.0%.

Further, Mo is an element which has a function of increasing resistance against pitting by Cl⁻, and it is desirable to set the content of Mo to 0.2% or more to acquire such an advantageous effect. On the other hand, when the content of Mo exceeds 2.0%, the strength of the stainless steel is lowered and, at the same time, a manufacturing cost sharply rises. Accordingly, the content of Mo is preferably limited to 2.0% or less. The content of Mo is more preferably limited to a value which falls within a range from 0.2 to 1.0%.

Group B: one kind or two or more kinds selected from 0.20% or less V, 0.10% or less Ti, 0.005% or less B.

All of V, Ti and B are elements which increase strength of the stainless steel, and the stainless steel may selectively contain one kind or two or more kinds of these elements when necessary.

To acquire such an advantageous effect, it is desirable that the stainless steel contains 0.02 or more V, 0.02% or more Ti, 0.0015% or more B. On the other hand, when the content of V exceeds, 0.20%, the content of Ti exceeds 0.10%, or the content of B exceeds 0.005%, toughness is lowered. Accordingly, when the stainless steel contains these elements, it is desirable to set the content of V to 0.20 or less, the content of Ti to 0.10% or less, and the content of B to 0.005% or less. It is more preferable to set the content of V to 0.02 to 0.10%, the content of Ti to 0.02 to 0.05%, and the content of B to 0.0015 to 0.0040%.

A balance of the stainless steel except for the above-mentioned components is formed of Fe and unavoidable impurities. As unavoidable impurities, 0.010% or less of 0 is allowable.

Next, the structure of seamless steel tube for OCTG is explained.

The seamless steel tube for OCTG has the structure which is mainly in a tempered martensite phase and in which precipitated Nb is dispersed. As the structure other than the tempered martensite phase, the structure may contain 5 volume % or less of a delta ferrite and 5 volume % or less of austenite respectively. Further, from a viewpoint of workability, the content of delta ferrite may preferably be set to 2 volume % or less. Also from a viewpoint of stability of strength, the content of austenite may preferably be set to 2 volume % or less. Due to such structure, it is possible to provide a steel tube which has desired high strength (yield strength: 95 ksi or more) and high toughness (fracture transition temperature vTrs in a Charpy impact test: −40° C. or below) and, at the same time, also possesses sufficient corrosion resistance as a tube for oil country tubular goods. A precipitated Nb quantity is set to 0.020 mass % or more in terms of Nb. When the precipitated Nb quantity is less than 0.020 mass %, the grain boundary precipitation of M₂₃C₆ type Cr carbide which adversely influences toughness cannot be suppressed so that toughness is lowered. The precipitated Nb quantity is preferably set to 0.025 mass % or more in terms of Nb. The seamless steel tube for OCTG does not contain M₃c type Cr type carbide.

The precipitated Nb quantity is determined such that an electrolytic residue obtained by electrolytic extraction using an electrolytic extraction method is subject to a chemical analysis, thus obtaining a quantity of Nb contained in the electrolytic residue, and the obtained quantity of Nb is used as the precipitated Nb quantity contained in a sample.

The precipitated Nb is mainly formed on Nb carbide or Nb carbonitride. The precipitated Nb is a precipitated material having a spherical shape with an average particle size of 3 nm to 15 nm.

Next, a manufacturing method of a seamless steel tube for OCTG is explained. Using the stainless steel seamless tube having the above-mentioned composition as a starting material, quenching and tempering are applied to the seamless steel tube. Further, straightening may also be applied to the seamless steel tube for straightening a deformed steel tube shape when necessary.

Although a manufacturing method of a starting material which has the above-mentioned composition is not particularly limited, it is preferable that molten steel having the above-mentioned composition is produced by a usually known melting method such as a steel converter, an electric furnace, a vacuum melting furnace, a molten state is formed into a steel tube raw material such as billets by a usual method such as continuous casting, ingot casting or blooming method. Then, these steel tube material is heated and is formed into a seamless steel tube having a desired size by hot working using manufacturing steps of a usual Mannesmann-plug mill method or a usual Mannesmann-mandrel mill method, and the seamless steel tube is used as a starting raw material. A seamless steel tube may be manufactured by hot extruding using a press method. Further, after forming the seamless steel tube, it is desirable to cool the seamless steel tube to a room temperature at a cooling rate of air cooling or more.

The starting material (seamless steel tube) is firstly subject to quenching.

Quenching implies treatment in which the seamless steel tube is reheated to a quenching temperature of A₃ transformation temperature or above and, thereafter, the seamless steel tube is cooled to a temperature zone of 100° C. or below from the quenching temperature at a cooling rate of air cooling or more. Due to such quenching, it is possible to form the structure of the starting material into the fine martensitic structure. When the quenching heating temperature is below the A_(c3) transformation temperature, the temperature of the seamless steel tube cannot be heated at a austenite single phase region so that the sufficient martensitic structure cannot be formed by cooling after heating. Hence, the seamless steel tube cannot ensure desired strength (yield strength: 95 ksi or more). Accordingly, the heating temperature of quenching is limited to an A_(c3) transformation temperature or above. The heating temperature is preferably set to 1000° C. or below.

Further, cooling from the quenching heating temperature is performed until a temperature zone of 100° C. or below at a cooling rate of air cooling or more. The starting material possesses the high quenching property. Hence, when the starting material is cooled down to the temperature zone of 100° C. or below at a cooling rate of approximately air cooling, the seamless steel tube can acquire the sufficient quenching structure (martensitic structure). Further, it is preferable to set a holding time of the starting material at the quenching temperature to 10 minutes or more from a viewpoint of homogeneous heating.

The seamless steel tube to which quenching is applied is subsequently subject to tempering.

Tempering is an important treatment for ensuring excellent low-temperature toughness. In the tempering, the seamless steel tube is heated to a tempering temperature which is 550° C. or more, and preferably A_(c1) transformation temperature or below, the heating is preferably held for 30 minutes or more and, thereafter, the seamless steel tube is cooled down preferably to a room temperature preferably at a cooling rate of air cooling or more. Due to such tempering, it is possible to produce the seamless steel tube which possesses both high strength of YS of 95 ksi or more and the excellent low-temperature toughness of vTrs of −40° C. or below. When the temperature at straightening is set to a tempering temperature or above, the structure is changed. Hence, there is no way but to set the temperature at straightening to the tempering temperature or below when the tempering temperature is less than 550° C. Accordingly, as described later, irregularities of yield strength YS are liable to occur. On the other hand, when the tempering temperature exceeds the A_(c1) transformation temperature, an austenite phase is formed and the austenite phase is transformed into quenched martensite at the time of cooling. Since the quenched martensite has many mobile dislocations, when the quenched martensite precipitates, yield strength YS is lowered. Further, from a viewpoint of acquiring the sufficient martensite, it is preferable to perform cooling from the tempering temperature at a cooling rate of air cooling or more.

Further, when necessary, straightening may be applied to the seamless steel tube for straightening the deformed steel tube shape following the tempering. It is preferable to perform straightening in a temperature zone of 450° C. or above. When the temperature at the straightening is less than 450° C., a working strain is locally generated in the steel tube at the time of performing straightening so that irregularities are liable to occur in mechanical properties, and particularly yield strength YS. Accordingly, when straightening is performed, the straightening is performed within a temperature zone of 450° C. or above. The desired irregularities (ΔYS) of yield strength YS is 15 MPa or less.

The seamless steel tube manufactured by the above-mentioned manufacturing method is formed into the martensitic stainless steel seamless tube which has the above-mentioned composition and structure, and possesses high strength of yield strength of 95 ksi or more (655 MPa or more) and the excellent low-temperature toughness of fracture transition temperature vTrs of −40° C. or below in a Charpy impact test, and further possesses the sufficient corrosion resistance as the oil country tubular goods.

EXAMPLES

The molten steel having the composition shown in Table 1 is degassed and, thereafter, billets (size: 207 mmφ) are formed by casting by a continuous casting method and are used as steel tube materials. These steel tube materials are heated, and are formed into tubes by hot working through Mannesmann-method manufacturing steps and, thereafter, the tubes are air-cooled so as to form seamless steel tubes (outer diameter of 177.8 mmφ×wall thickness of 12.65 mm).

Specimens (steel tubes) are sampled from the obtained seamless steel tubes, and quenching and tempering are applied and optional straightening is further applied to the specimens (steel tubes) under conditions shown in Table 2 and Table 3.

Electrolytic extraction specimens are sampled from the specimens (steel tubes) to which quenching and tempering are applied and optional straightening is further applied. Using the sampled electrolytic extraction specimen, a quantity of Nb contained in an obtained electrolytic residue is obtained using an electrolytic extraction method and is determined as a quantity of precipitated Nb contained in the specimen.

Further, from the specimen (steel tubes) to which quenching and tempering are applied and optional straightening is further applied, a strip specimen specified by API standard 5CT is sampled, and a tensile test is carried out on the strip specimen thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimen. With respect to the specimen (steel tube) to which the straightening is applied, an increment ΔYS of YS by straightening is obtained. The tensile test is carried out on the non-straightened steel tubes manufactured under the same condition except for straightening, thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimens. An increment ΔYS of YS by straightening is calculated by a following formula:

ΔYS=(YS of straightened steel tube)−(YS of non-straightened steel tube).

Further, V-notch specimens (thickness: 10 mm) are sampled from specimens to which quenching and tempering are applied and optional straightening is further applied in accordance with the stipulation of JIS Z 2242. The V-notch specimens are subject to a Charpy impact test where a fracture transition temperature vTrs is obtained and toughness is evaluated.

Further, a corrosion specimen having a thickness of 3 mm, a width of 30 mm and a length of 40 mm is prepared by machining from the specimens, and a corrosion test is carried on the corrosion specimen.

The corrosion test is carried out in such a manner that the corrosion specimen is immersed into a test solution: 20% NaCl aqueous solution (temperature of the test solution: 80° C., under CO₂ gas atmosphere of 30 atmospheric pressure) held in an autoclave, and an test duration is 1 week (168 h). With respect to the specimen after the corrosion test, the weight of the specimen is measured and a corrosion rate is calculated based on the reduction of weight before and after the corrosion test.

An obtained result of the test is shown in Table 4 and Table 5.

All of our examples (steel tubes No. 4 to 13, 18 to 23, 29 and 30) are martensitic stainless steel seamless tubes which exhibit the sufficient corrosion resistance, possess both of high strength of YS of 95 ksi or more (655 MPa or more) and excellent low-temperature toughness of vTrs of −40° C. or less, and allows hot straightening at a temperature of 450° C. or more so that an increment of yield strength is small and the difference ΔYS of the average YS is small (15 MPa or less) even when straightening is applied. On the other hand, with respect to comparison examples (steel tubes No. 1 to 3, 14 to 17, 24 to 28) which are outside our range, strength is insufficient (YS: less than 95 ksi) or the low-temperature toughness is low (vTrs exceeding −40° C.) so that desired high strength and desired high toughness cannot be ensured. Further, an increment of yield strength (ΔYS exceeding 15 MPa) after straightening is increased.

TABLE 1 Chemical component (mass %) Formula (1)* Left Steel side No. C Si Mn P S Al Cr Ni N Nb value Appropriate Cu, Mo V, Ti, B Remarks A 0.026 0.16 1.20 0.016 0.0011 0.021 11.0 1.5 0.025 0.04 −0.29 x Cu: 0.5 V: 0.02 Comparison Example B 0.010 0.12 1.14 0.019 0.0010 0.039 10.9 2.3 0.005 — −0.07 x Cu: 0.51 — Comparison Example C 0.010 0.15 1.21 0.016 0.0009 0.022 10.9 2.0 0.008 0.04 −0.33 ∘ Cu: 0.5 V: 0.03 Example D 0.009 0.15 1.23 0.003 0.0010 0.025 11.1 2.0 0.009 0.04 −0.35 ∘ Cu: 0.5 V: 0.03 Example E 0.009 0.15 1.23 0.017 0.0007 0.024 11.1 2.0 0.010 0.04 −0.34 ∘ Cu: 0.5 V: 0.07 Example F 0.011 0.15 1.21 0.017 0.0007 0.030 10.9 2.0 0.010 0.07 −0.59 ∘ Cu: 0.5 V: 0.03 Example G 0.009 0.16 1.20 0.017 0.0008 0.030 11.1 2.0 0.010 0.04 −0.36 ∘ Cu: 0.5 V: 0.03, Example Ti: 0.035 H 0.025 0.19 2.50 0.017 0.0020 0.024 13.2 — 0.066 0.08 −0.60 ∘ — V: 0.03 Comparison Example I 0.011 0.16 1.23 0.017 0.0020 0.023 11.1 2.0 0.025 — −0.01 x Cu: 0.5 V: 0.03, Comparison Example B: 0.0028 J 0.025 0.20 2.02 0.017 0.0011 0.025 11.0 — 0.060 — −0.04 x Cu: 2.5 V: 0.03 Comparison Example K 0.010 0.15 1.20 0.015 0.0010 0.025 11.0 2.5 0.010 0.10 −0.81 ∘ Cu: 0.5 V: 0.03 Example L 0.010 0.15 1.70 0.015 0.0010 0.025 11.0 1.6 0.010 0.14 −1.12 ∘ Cu: 0.5 V: 0.03 Example M 0.010 0.15 1.20 0.015 0.0010 0.025 11.0 2.0 0.010 0.07 −0.58 ∘ Cu: 0.5, V: 0.03 Example Mo: 0.2 N 0.010 0.15 1.20 0.015 0.0010 0.025 11.0 2.5 0.010 0.07 −0.58 ∘ Cu: 0.5 V: 0.13 Example O 0.030 0.25 0.52 0.013 0.0009 0.004 10.8 1.2 0.027 — −0.05 x Cu: 1.0, V: 0.04 Comparison Example Mo: 0.2 P 0.040 0.22 0.66 0.016 0.0011 0.001 11.6 1.7 0.051 0.02 −0.07 x — V: 0.04, Comparison Example Ti: 0.10, B: 0.001 Q 0.017 0.17 1.24 0.015 0.0011 0.023 11.4 2.0 0.020 0.04 −0.28 x Cu: 0.5 V: 0.04 Comparison Example R 0.008 0.21 0.84 0.015 0.0010 0.020  9.4 2.3 0.010 — −0.03 x — — Comparison Example S 0.008 0.18 1.21 0.015 0.0010 0.030 14.5 3.5 0.010 — −0.05 x Cu: 0.5 — Comparison Example T 0.015 —** —** —** —** 0.023 11.3 2.1 0.020 0.04 −0.32 ∘ — — Example U 0.010 —** —** —** —** —** 11.0 1.5 0.020 0.05 −0.32 ∘ — — Example *C − 31/4Nb + 7/6N − 9/4Al ≦ −0.30 **not added

TABLE 2 Quenching Tempering Cooling Cooling Straightening Steel Heating stop Tempering stop Straightening tube Steel temperature temperature temperature temperature temperature No. No. (° C.) Cooling (° C.) (° C.) Cooling (° C.) (° C.) Remarks 1 A 850 air cooling 25 550 air cooling 25 — Comparison Example 2 A 850 air cooling 25 650 air cooling 25 — Comparison Example 3 B 810 air cooling 25 500 air cooling 25 400 Comparison Example 4 C 850 air cooling 25 650 air cooling 25 — Example 5 C 850 air cooling 25 675 air cooling 25 — Example 6 C 850 air cooling 25 650 air cooling 25 550 Example 7 D 850 air cooling 25 650 air cooling 25 — Example 8 E 850 air cooling 25 650 air cooling 25 — Example 9 F 850 air cooling 25 600 air cooling 25 — Example 10 F 850 air cooling 25 700 air cooling 25 — Example 11 F 850 air cooling 25 625 air cooling 25 — Example 12 F 850 air cooling 25 600 air cooling 25 500 Example 13 G 850 air cooling 25 650 air cooling 25 — Example 14 H 850 air cooling 25 600 air cooling 25 — Comparison Example 15 H 850 air cooling 25 700 air cooling 25 — Comparison Example 16 I 850 air cooling 25 600 air cooling 25 — Comparison Example

TABLE 3 Quenching Tempering Cooling Cooling Straightening Steel Heating stop Tempering stop Straightening tube Steel temperature temperature temperature temperature temperature No. No. (° C.) Cooling (° C.) (° C.) Cooling (° C.) (° C.) Remarks 17 J 850 air cooling 25 600 air cooling 25 — Comparison Example 18 K 850 air cooling 25 600 air cooling 25 — Example 19 K 850 air cooling 25 600 air cooling 25 525 Example 20 L 850 air cooling 25 600 air cooling 25 — Example 21 L 850 air cooling 25 600 air cooling 25 510 Example 22 M 850 air cooling 25 600 air cooling 25 — Example 23 N 850 air cooling 25 600 air cooling 25 — Example 24 O 920 air cooling 25 350 air cooling 25 300 Comparison Example 25 P 920 air cooling 25 400 air cooling 25 350 Comparison Example 26 Q 850 air cooling 25 600 air cooling 25 — Comparison Example 27 R 850 air cooling 25 550 air cooling 25 — Comparison Example 28 S 850 air cooling 25 550 air cooling 25 — Comparison Example 29 T 850 air cooling 25 675 air cooling 25 — Example 30 U 850 air cooling 25 675 air cooling 25 — Example

TABLE 4 Precipitate(s) Tensile characteristic Corrosion Precipitated Nb Increment of resistance Steel amount YS by Toughness Corrosion tube Steel (in terms of Nb) YS TS straightening vTrs rate No. No. (mass %) (MPa) (MPa) ΔYS (MPa) (° C.) (mm/y) Remarks 1 A 0.024 825 1010 —   10 0.05 Comparison Example 2 A 0.025 716 840 — −35 0.05 Comparison Example 3 B — 870 903 33 −75 0.03 Comparison Example 4 C 0.028 738 823 — −40 0.035 Example 5 C 0.029 661 815 — −70 0.035 Example 6 C 0.028 740 823  2 −40 0.035 Example 7 D 0.026 738 823 — −40 0.035 Example 8 E 0.024 759 839 — −50 0.035 Example 9 F 0.052 838 873 — −50 0.035 Example 10 F 0.054 712 869 — −100  0.035 Example 11 F 0.053 795 853 — −50 0.035 Example 12 F 0.052 848 878 10 −50 0.035 Example 13 G 0.027 679 723 — −50 0.035 Example 14 H 0.056 753 892 — −20 0.06 Comparison Example 15 H 0.058 642 771 — −60 0.06 Comparison Example 16 I — 698 791 —   10 0.035 Comparison Example

TABLE 5 Precipitate(s) Tensile characteristic Corrosion Precipitated Nb Increment of resistance Steel amount YS by Toughness Corrosion tube Steel (in terms of Nb) YS TS straightening vTrs rate No. No. (mass %) (MPa) (MPa) ΔYS (MPa) (° C.) (mm/y) Remarks 17 J — 812 930 —   10 0.10 Comparison Example 18 K 0.071 849 875 — −50 0.03 Example 19 K 0.071 852 875  3 −50 0.03 Example 20 L 0.102 863 894 — −55 0.05 Example 21 L 0.102 868 897  5 −55 0.05 Example 22 M 0.049 838 873 — −60 0.04 Example 23 N 0.048 896 923 — −55 0.03 Example 24 O — 855 1053 47 −50 0.06 Comparison Example 25 P 0.013 833 1094 42 −55 0.04 Comparison Example 26 Q 0.022 789 877 — −30 0.04 Comparison Example 27 R — 680 761 — −30 0.04 Comparison Example 28 S — 699 803 — −25 0.01 Comparison Example 29 T 0.029 670 821 — −60 0.04 Example 30 U 0.030 660 807 — −60 0.03 Example 

1-12. (canceled)
 13. A martensitic stainless steel seamless tube for oil country tubular goods having a yield strength of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.
 14. A martensitic stainless steel seamless tube for oil country tubular goods having yield strength YS of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.
 15. The martensitic stainless steel seamless tube according to claim 13, wherein the composition satisfies formula (I): C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
 16. The martensitic stainless steel seamless tube according to claim 13, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 17. The martensitic stainless steel seamless tube according to claim 13, wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.20% or less V, 0.10% or less Ti and 0.005% or less B.
 18. A method of manufacturing a martensitic stainless steel seamless tube for oil country tubular goods, comprising: quenching a stainless steel seamless tube having a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance by heating at a quenching temperature of A_(c3) transformation temperature or above and, thereafter, cooling the seamless tube to a temperature range of 100° C. or less from the quenching temperature at a cooling rate of air cooling or more; and tempering the seamless tube by heating at a tempering temperature of 550° C. or more and A_(c1) transformation temperature or below and then cooling.
 19. A method of manufacturing a martensitic stainless steel seamless tube for oil country tubular goods, comprising: quenching a stainless steel, seamless tube having a composition comprising, by mass %, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance by heating at a quenching temperature of A_(c3) transformation temperature or above and, thereafter, cooling the seamless tube to a temperature range of 100° C. or less from the quenching temperature at a cooling rate of air cooling or more; and tempering the seamless tube by heating at a tempering temperature of 550° C. or more and A_(c1) transformation temperature or below and then cooling.
 20. The method according to claim 18, wherein the composition satisfies formula (I): C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
 21. The method according to claim 18, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 22. The method according to claim 18, wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.10% or less V, 0.10% or less Ti and 0.005% or less B.
 23. The method according to claim 18, wherein straightening is performed in a temperature range of 450° C. or more during cooling after said tempering.
 24. A martensitic stainless steel seamless tube for oil country tubular goods obtained by the method according to claim 23, wherein an increment ΔYS of yield strength by the straightening is 15 MPa or less.
 25. The martensitic stainless steel seamless tube according to claim 14, wherein the composition satisfies formula (I): C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
 26. The method according to claim 19, wherein the composition satisfies formula (I): C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
 27. The martensitic stainless steel seamless tube according to claim 14, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 28. The martensitic stainless steel seamless tube according to claim 15, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 29. The method according to claim 19, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 30. The method according to claim 20, wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo.
 31. The martensitic stainless steel seamless tube according to claim 14, wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.20% or less V, 0.10% or less Ti and 0.005% or less B.
 32. The method according to claim 19, wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.10% or less V, 0.10% or less Ti and 0.005% or less B. 